A hierarchical storage system in which a plurality of storage mediums (storage devices) are used in combination with each other is sometimes used as a storage system that stores data. As the plurality of storage mediums, solid state drives (SSD's), which are capable of high-speed access but have a comparatively small capacity and a high cost, and hard disk drives (HDD's), which have a comparatively large capacity and a low cost but a comparatively low speed, may be used.
In a hierarchical storage system, it is possible to increase the use efficiency of the SSD's and increase the performance of the entire system, by arranging data of a storage region having a low access frequency in a HDD and arranging data of a storage region having a high access frequency in an SSD. Therefore, in order to improve the performance of a hierarchical storage system, it is preferable to efficiently arrange data of a region having a high access frequency in an SSD.
As an example of a method of arranging data having a high access frequency in an SSD, a method is known in which regions having a high access frequency are arranged in an SSD in units of 1 day in accordance with the access frequency of the preceding day, for example.
However, in a pattern of access to a storage system used in file sharing etc., input/output (IO) requests (hereafter may be simply referred to as IO's) may be concentrated in a narrow range in the storage region in comparatively short period of time of around several minutes to several tens of minutes and then the IO's may move to another region as time passes. For such a workload, it is often the case that the workload may not be tracked by totaling the access frequencies for a long period of, for example, 1 day. “Workload” is an index that represents the distribution of accesses to a storage device (usage state of storage device) and the workload changes with the passing of time and an offset position (storage region) of the storage device.
As an example of a technique for handling a workload where the load moves in a short period of time, a technique is known in which the occurrence of IO concentration is monitored and whenever IO concentration occurs, the region in which the IO concentration has occurred is moved from a HDD to an SSD. In this technique, the region that was moved to the SDD is moved back to the HDD once the IO concentration has ended, the SSD region is freed up, and consequently a region where IO concentration next occurs can be quickly moved to the SSD. In addition, a technique is also known in which a region obtained by linking together the regions in the vicinity of a high load region is determined to be a movement target region at this time.
As examples of the related art, Japanese Laid-open Patent Publication No. 2014-164510, Japanese Laid-open Patent Publication No. 2014-191503, Japanese Laid-open Patent Publication No. 2014-229144, Japanese Laid-open Patent Publication No. 2013-171305 and Japanese Laid-open Patent Publication No. 9-214935 are known.